The present invention relates to a method for operating a load supplied from an a-c network via an intermediate-link converter in the event of a network disturbance until the current in the intermediate link has decayed, especially wherein the load is an asynchronous machine. The invention also relates to apparatus for carrying out the method.
The invention is explained for the preferred case wherein an asynchronous machine is provided as the load. Other loads, however, also can be used which generate at the converter output a periodic counter-voltage, the amplitude of which decays only slowly even if the converter is blocked.
If the a-c voltage of the supplying network which may have three or more phases throughout, break downs or if amplitude or frequency disturbances occur, the converter must be shut off so that no faulty commutations occur. The machine then is without current rapidly and the machine flux decays slowly (exponentially). When the power returns, however, an appreciable torque can be generated only if an appropriate flux with a suitable phase relative to the current is available.
If, upon reconnecting the network to the still running machine, the desired flux is to be built up again as quickly as possible, therefore the frequency at the machine-side output of the converter ("machine frequency") must correspond to the machine speed during the reconnect action. To this end, the machine frequency can be slaved to the machine speed during the interruption of the network via a tachometer. This requires, on the one hand, a tachometer and, on the other hand, problems can arise in the synchronized reconnection to the still excited machine at machine frequencies which are higher than the network frequency. Synchronized connection of the network is generally made more difficult by the circumstance that, after a disturbance, the network is initially asymmetrical and requires a certain amount of time to become symmetrical after the reconnection.
In addition, it is frequently undesirable that the machine flux decays steadily during the disturbance and can be built up again only after the network disturbance.
Other methods, in which, for instance, the machine is separated from the converter after the network interruption and is connected directly to the network, or in which the machine frequency is slowly increased starting from zero according to a ramp function until the frequency and the speed coincide, are frequently eliminated for technological reasons.